1. Field of Invention
This invention relates to the fields of plasma generation, ignitions, and internal combustion (IC) engines. In particular, it relates, but is not limited, to ignition methods and ignition apparatus for use therein; and, specifically, to ignition methods and apparatus for various applications, including but not limited to, high pressure engines. More particularly, some aspects relate to the delivery of discharge current to traveling spark igniters in order to maximize their performance and longevity, especially in internal combustion engines operating at high pressures.
2. Discussion of Related Art
For a variety of reasons, there is interest today in increasing the pressures in internal combustion engines and similar combustion environments, with a concomitant need for ignition sources capable of operating in these environments. For example, automobile companies and manufacturers of internal combustion engines would like to be able to provide vehicles which have IC engines which operate at much higher pressures than conventional internal combustion engines. To date, however, there has not been an effective and practical ignition system for such engines. Among other concerns are longevity of igniters (spark plugs) and reliability of igniter firing.
The traveling spark igniter (TSI) is a device that has been discussed as a promising spark plug replacement for internal combustion engines, but previously not for high pressure engines. TSIs have, for example, been shown in a number of prior patents including, for example, U.S. Pat. Nos. 6,321,733 and 6,474,321, both assigned to the same assignee as this invention and incorporated by reference in their entireties for their explanations of TSI devices and ignition systems.
Briefly, a TSI-based ignition system provides a large plasma kernel which is propagated along the igniter's electrodes by Lorentz force (along with thermal forces, to lesser degrees) and propelled into a combustion chamber. The Lorentz force acting on the ignition kernel (i.e., plasma) is created by way of the discharge current in the plasma interacting with a magnetic field caused by that same current in the electrodes of the igniter. The magnitude of the Lorentz force is proportional to the square of that current. In engines operating at normal pressures (i.e., a maximum of about 120 psi), traveling spark igniters provide significant advantages over conventional spark plugs due to the large plasma volume they generate, typically some 100-200 times larger than in a conventional spark plug, for comparable discharge energy. Increased efficiency and reduced emissions are attainable.
For higher engine operating pressures, however, the breakdown voltage required for initiating the discharge between the electrodes of the igniter is significantly higher than in engines operating at conventional pressures. This creates problems for TSIs, as for any spark plug. The electrodes in a TSI, as in a conventional spark plug, are maintained in a spaced apart relationship by a member called an isolator, which is formed of an insulating material such as a ceramic. The higher breakdown voltage causes problems for both the isolator and the electrodes.
Along the surface of the isolator running between the electrodes, the breakdown voltage is lower than it is further along the electrodes in a TSI, or in any conventional spark plug with a similar gap between the electrodes. Indeed, this difference in breakdown voltages varies directly with increasing pressure in the combustion chamber. Consequently, although the breakdown voltage along the isolator surface increases with pressure, that increase is less than the increase in the breakdown voltage between the exposed part of the electrodes away from the isolator surface. When breakdown occurs (as a result of which the resistance through the plasma rapidly drops), the current rises rapidly and a very large current is conducted in the forming plasma along the isolator surface, thus giving rise to the Lorentz force acting on the plasma. Such rapidly rising current, though, creates not only a very high temperature plasma, but also a powerful shock wave in the vicinity of the surface of the isolator. The larger the current, the more rapid the plasma expansion and the resulting shock wave. These combined effects can cause deformation and/or breakage of the isolator.
Additionally, the high current produces very rapid erosion of the electrodes in the vicinity of the isolator surface, where they are attacked by the high current, thermal heating and thermionic emission that results therefrom.
Similar problems have been manifest with igniters based on the University of Texas “railplug” design which generates a Lorentz force in a plasma traveling along a high aspect ratio discharge gap (as contrasted with a TSI, which has a low aspect ratio discharge gap).
Although both the railplug and the TSI generate significant plasma motion at relatively low pressures, when the combustion chamber pressure is increased to a high pressure, the plasma behaves differently and it is this difference in behavior that leads to unsatisfactory results. In a low pressure environment, the force exerted on the plasma by the pressure is relatively small. The plasma moves easily along the electrodes in response to the Lorentz force. As the ignition chamber pressure is increased, however, that pressure provides a force of significant magnitude that resists the Lorentz force and, thus, plasma motion. Consequently, the plasma tends to become more concentrated, and to collapse on itself; instead of having a diffused plasma cloud, a very localized plasma—an arc—is formed between the electrodes below a certain current threshold. This arc, though occupying a much smaller volume than the plasma cloud of the low-pressure case, receives similar energy. As a result, the current density is higher and at the electrodes, where the arc exists, there is a higher localized temperature and more power density at the arc-electrode interfaces. That is, the current density is quite high at those interfaces, producing more localized heating of the electrodes than in the low pressure environment. The localized heating of the electrodes, in turn, produces thermionic emission of electrons and ions. The observed effect is that the arc appears to “attach” itself at relatively fixed locations on the electrodes, producing erosion of the electrodes as the entire discharge energy is deposited at the “attachment point;” this is to be contrasted with the low pressure environment where a lower density, diffused area of plasma contact moves along the electrodes without significantly damaging them.
Concurrently, the plasma, affected by the Lorentz and thermal forces, bows out from the arc attachment points. This causes the magnetic field lines to no longer be orthogonal to the current flow between the electrodes, reducing the magnitude of the Lorentz force produced by a given current. So, in addition to the other problems, there is a loss in motive force applied to the plasma.
Overall, there is a reduction in plasma motion as compared with the lower pressure environments, and dramatically increased electrode wear at the arc attachment points.
Accordingly, a variety of needs exist, including needs for plasma generators, in general, needs for improved ignition systems, needs for ignition systems for use in internal combustion engines and needs for an ignition system and method which generates a large ignition kernel and which is usable with high pressure engines, and is commercially practical.
If a traveling spark igniter is to be used in a high pressure combustion environment, a need further exists to overcome the above negative effects on the isolator material and electrodes of the igniter. See U.S. Pat. Nos. 5,704,321, 6,131,542, 6,321,733, 6,474,321, 6,662,793, and 6,553,981, for example, incorporated by reference herein. That is, a need exists for an igniter and ignition system for use in high pressure combustion engines, wherein the isolator and electrodes exhibit substantial lifetimes (preferably comparable to that of conventional spark plugs in low pressure engines) without being destroyed by the discharge process. Desirably, such a traveling spark igniter and ignition system will be usable and useful in internal combustion engines operating not only at high and very high pressures (i.e., several hundred psi), but also at lower, conventional pressures.